1. Field of the Invention
This invention relates to a tire for construction vehicles with regular depth in the tread grooves including the standard aize of 16.00-25 or larger and the wide size of 20.5-25 or larger, and more particularly to a tire for high-speed construction vehicles with a bias-ply structure.
2. Prior Art
In general the tread patterns of a tire for construction vehicles are classified into three types; i.e., the regular type with regular depth in the grooves, the deep type with depth approximately 1.5 times deeper than the regular type, the extra-deep type with depth approximately 2.5 times deeper than the regular. For a high-speed vehicle, the regular type is considered to be the best, since the deep type and the extra-deep type have such a structure that the tread thereof is too thick for high-speed operation. As a construction vehicle, its tire has to have a good pulling capacity; therefore, the tread pattern has to be designed to this effect. More specifically, the rib type having circumferential grooves in the tread is not appropriate because of the lack of pulling capacity; the lug type having lateral grooves, the block type having grooves both lateral and circumferential forming a number of blocks, and the semi-lug type which is a combination of the rag type and the rib type are more suited for construction vehicles.
As to the carcass of tires, the radial ply structure is typically used for high-speed purpose. However, because of insufficient rigidity, the total operability as a construction vehicle sometimes is not satisfactory depending upon the operating conditions such as the road and weather conditions. On the other hand, the bias-ply structure tends to creat high temperature inside. Therefore, particularly when used for high-speed purpose, it is likely that the tire cause separation because of heat generated during high-speed running. In other words, the bias ply has a higher heat generating rate than the radial ply, since the ply cords of the former have more changes in the angle by compression and expansion from loading and unloading during rotation. Thus, the bias ply is considered to be unsuitable for high-speed running. Accordingly, there have been a number of proposals in an attempt to improve the heat-proof quality; for example, varying the mixing rate of tread gum, varying the design of the carcass to cut heat generation to an extent, and decreasing the thickness of the undertread portion, namely, the thickness between the bottom of the grooves and the surface of the carcass to improve its heat releasing ability. Those attempts, however, have improved the heat-proof quality only 30 percent utmost and were not successful solution to the problem.